1. Field of the Invention
The present invention relates to hydraulic systems for vehicles and, more particularly, to a hydraulic system having a hydraulic fluid pump and at least one hydraulic device.
2. Description of the Related Art
Many trucks with hydraulic braking systems, particularly larger gasoline powered and diesel powered trucks, incorporate hydraulic braking assist systems, rather than vacuum assist systems commonly found in passenger automobiles. The use of vacuum assist braking systems can be problematic in vehicles having a turbo-charged engine and such vehicles will also often employ hydraulic braking assist systems. Furthermore, there is an aftermarket demand for hydraulic braking assist systems for vehicles, such as hotrods, that may not otherwise have a brake assist device or for which the use of a vacuum assist system presents difficulties. Such hydraulic braking assist systems are well known and sold commercially.
Typically, these hydraulic braking assist systems are connected in series between the steering gear and hydraulic pump and use flow from the pump to generate the necessary pressure to provide brake assist as needed. The flow from the pump is generally confined within a narrow range of flow rates and is not intentionally varied to meet changing vehicle operating conditions. Because of the series arrangement, the application of the brakes and engagement of the hydraulic braking assist system can affect the flow of hydraulic fluid to the steering gear, thereby affecting the amount of assist available to the steering gear. Specifically, when a heavy braking load is applied, it causes an increase in backpressure to the pump which can exceed a threshold relief pressure (e.g., 1,500 psi) of the pump. Above this level, a bypass valve of the pump opens to divert a fraction of the outflow back to the intake of the pump, where the cycle continues until the pressure from the brake assist device drops below the threshold value of the bypass valve. During this relief condition, a diminished flow of fluid is sent to the steering gear which may result in a detectable increase in steering effort by the operator of the vehicle to turn the steering wheel under extreme relief conditions.
To at least partially alleviate this condition, it is possible to place a flow-splitter or priority valve in the hydraulic system to divert a portion of the flow of fluid being discharged from the pump to the steering gear under heavy braking conditions. The disclosure of U.S. Pat. No. 6,814,413 B2 describes the use of such a flow-splitter and hereby incorporated herein by reference. While the use of a flow splitter to divert a portion of the fluid flow being discharged by the pump to the steering gear assist device during heavy braking conditions provides significant advantages, the use of such a flow splitter generally requires that the steering gear device have a pressure relief value that is at least large as the pressure relief value of the brake assist device.
Oftentimes, a steering gear assist device that is adequate for a particular vehicle will have a lower pressure relief value than the brake assist device required for that same vehicle. Thus, the requirement that the steering gear assist device have a pressure relief value that is at least as large as the pressure relief value of the brake assist device often has a direct impact on the selection of a steering gear assist device and results in the selection of a more expensive steering gear assist device.